The present invention relates generally to bicycles and, more particularly, to a light weight multi-material fork and steerer tube assembly.
A typical fork assembly generally includes a fork crown that is constructed to engage a pair of downward extending forks. A steerer tube is constructed to engage mating structure and be secured to the fork crown so as to extend in a direction generally opposite the pair of forks. Typically, the steerer tube and fork crown are constructed of aluminum or metal-type materials whereas the forks, or at least a portion thereof, may be constructed of a composite material and/or a carbon based material such as carbon fiber material and/or a glass fiber material. The fork crown is often two dimensionally forged and then machined to a proximate finish or net shape. The fork crown commonly extends in opposite lateral directions relative to the axis of the steerer tube.
The fork crown is frequently formed with a pair of protrusions positioned on generally opposite sides of the fork crown relative to the steerer tube. The protrusions are commonly constructed to engage the inside of the composite fork legs. The faces of the protrusions increase the surface area of the interface between the aluminum or metal material fork crown and the composite forks. Such a construction provides a greater bonding area between the two components.
Once fully assembled and bonded, the assembly is again machined to ensure a generally smooth transition between the fork crown and the fork legs thereby providing an aesthetic and aerodynamic finish. In addition to the exterior surface machining, a surface of a cavity of each fork leg is also commonly machined to ensure a relatively consistent bond-gap between a respective fork leg and the respective protruding portion of the fork crown.
Each fork blade or leg is typically made from a carbon fiber and/or glass fiber material that is held together with an epoxy resin matrix. Such fork blades are typically molded using matched female tooling and a pressure-generating material or pressurized bladder that is configured to form the general shape of the cavity of each fork leg such that each cavity is configured to snuggly receive the corresponding protrusion of the fork crown. This construction and preparation of such a fork assembly and the fork assembly components is time consuming and labor intensive.
Construction of the steerer tube also commonly requires extensive manufacturing processes to ensure a secure engagement between the steerer tube and the fork crown. An inner diameter of the steerer tube is commonly stepped or tapered and is formed using a butting process that is well-known to steerer tube manufacture. The steerer tube also includes a plug end that is constructed for bonding the steerer tube to the fork crown. The plug end is generally formed after the butting process and is typically done by swaging the end of the steerer tube that engages the fork crown.
Although such a known manufacturing and assembly process generates a fork assembly that is aesthetically pleasing and fairly robust, such fork assemblies are not without their drawbacks. The assembly provides a relatively heavy fork assembly having a fork crown and steerer tube constructed of a relatively solid aluminum material. The fork crown and steerer tube are commonly constructed of metal-type materials and sized to withstand the stresses and strains associated with bicycle operation. The size and material of the steerer tube assembly undesirably contributes to the overall weight of the bicycle. Furthermore, due to stress concentrations associated with the interface of the steerer tube and the fork crown, additional material is commonly associated with this interface area thereby further undesirably increasing the mass of the fork assembly. Understandably, the weight of the steerer tube and fork subassembly is an important consideration of bicycle design. Riders commonly prefer a bicycle that is lightweight and can provide the performance to which they are accustomed.
The fairly complex manufacture of such fork assemblies also presents several undesirable manufacturing attributes. The multiple machining and complex forging, molding, or casting requirements of such assemblies increases the cost associated with producing each unit. Whereas the pre and post bond machining of the fork assembly components ensures a generally uniform and repeatable assembly, such manufacturing processes have a greater than ideal per unit cycle time. Although the post bond machining of the crown race ensures that the fork crown is constructed to be concentrically supported by a bicycle frame relative to the steerer tube, these extensive production procedures also increase the per unit assembly time as well as the requisite skill level of assembly and manufacturing personnel.
Therefore, it would be desirable to have a fork and steerer tube assembly that is both robust and lightweight. It is further desired to provide a method of forming a fork assembly whose components can be efficiently and repeatably produced and assembled.